1. Field of the Invention
The present invention relates to an improved pressure-operated switch for a high-voltage interrupting module. More specifically, the present invention relates to an improvement of the switches disclosed in commonly assigned U.S. Pat. Nos. 4,342,978, issued Aug. 3, 1982 in the name of Meister, and 4,370,531, issued Jan. 25, 1983 in the name of Tobin, and in the following commonly assigned U.S. patent applications: Ser. No. 179,367, filed Aug. 18, 1980 (now abandoned in favor of continuation application Ser. No. 550,201, filed Nov. 9, 1983) in the name of Jarosz and Panas; Ser. No. 179,366, filed Aug. 18, 1980 (now abandoned in favor of continuation application Ser. No. 539,396, filed Oct. 6, 1983) in the name of O'Leary, and Ser. No. 437,925, which issued Jan. 24, 1984 as U.S. Pat. No. 4,227,963, and 437,926 both filed Nov. 1, 1982 in the names of Jarosz and Panas.
2. Prior Art
The above patents and patent applications relate to various aspects of a pressure-operated switch and to a high-voltage interrupting module containing the switch. The switch may include a pair of contacts, which are normally electrically interconnected, for example, by direct abutment therebetween or, preferably, by interconnecting them with a shearable or tearable metallic disc or membrane. In preferred embodiments of the switch, one contact is stationary, while the other is movable, although both may be movable. The contacts are separable by their relative movement apart along a fixed line of direction to open a gap therebetween, thereby opening the switch. One of the contacts, preferably the stationary contact, contains a bore which, in conjunction with a piston or trailer between the movable contact and the bore, defines a closed chamber. The chamber houses a power cartridge or similar pressure-generating device.
The switch may be in electrical shunt with a fuse, a fusible element of which, as well as the switch, preferably reside within a common housing. When the switch is closed (i.e., when the contacts thereof are electrically interconnected), the resistance of the current path through the switch is much lower than the resistance of the current path through the fusible element, and, accordingly, a majority of the current flowing through the module flows through the switch. Thus, the module has a very high continuous current rating. Upon opening the switch, the contacts separate and current is rapidly commutated from the switch to the fusible element where it is interrupted. Separation of the contacts is achieved by igniting the power cartridge, which evolves high pressure within the chamber. This high pressure acts against the piston and the forces produced thereby rapidly drive the piston and the movable contact away from the stationary contact, which shears the disc to break the normal electrical interconnection and open the switch. The power cartridge may be ignited in response to a trip signal produced by apparatus which senses a fault current or other over-current in a circuit in which the interrupting module is connected for protection thereof. Such trip-signal-producing apparatus may be that which is disclosed in commonly assigned U.S. patent application, Ser. Nos. 506,942; 506,943; and 506,944, all filed June 22, 1983 in the name of Ruta.
In more specific embodiments of the switch described in the above patents and patent applications, a second stationary contact is included. When the switch is closed, the movable contact and the second stationary contact are electrically interconnected with a second shearable disc. When the power cartridge is ignited, movement of the movable contact also shears the second disc. As the movable contact moves away from the first stationary contact, it is telescoped into a bore formed in the second stationary contact. This bore may be lined with an insulative sleeve and the movable contact may be covered with an insulative sleeve, so that such telescoping results in the formation of a second gap, that is, a gap between the movable contact and the second stationary contact.
The movable contact moves rapidly away from the first stationary contact through a passageway in an insulative liner which the piston may also enter. The piston also enters the passageway in the liner to physically isolate the moving contact and the second stationary contact from the ignition products of the power cartridge. This isolation prevents or suppresses the formation of any arc between the separating contacts and between the stationary contacts. In preferred embodiments of the switch, the stationary contacts and the liner are engageably surrounded, and have their relative positions fixed, by an insulative housing, which maintains the stationary contact and the liner end-to-end with the bores and the passageway axially aligned.
Tests in low temperature environments (e.g., -40.degree.) of earlier versions of the switch (such as those disclosed in '978 and '531 patents and in the '367 and '366 applications) showed that, after the piston entered the liner, some of the ignition products of the power cartridge could flow along the housing-liner interface. Such flow could create the possibility of internal flashover of the open switch, i.e., undesired conduction between the stationary contacts. One object of the present invention is to eliminate such ignition product flow along the housing-liner interface.
It also appeared that in such earlier versions of the switch, ignition products could flow along the piston-liner interface, creating the possibility of internal flashover between the stationary contacts. On the assumption that this latter flow was caused by abrasion or distortion of the piston or the liner (or both) as the switch opened, both elements were made of abrasion-resistant, high surface lubricity, non-brittle, thus high molecular weight polyethylene (UHMWPE), as disclosed in the ]926 application. Tests of later versions of the switch showed that this ignition-product flow problem, though ameliorated by the UHMWPE piston and liner nevertheless could, in some cases, remain. Specifically, if manufacturing tolerances led to the passageway in the UHMWPE liner being too large or to the UHMWPE piston being too small, there could be sufficient clearance therebetween to permit flow of the ignition products therepast. Such flow could produce a conductive path between the first stationary contact and the second stationary contact. Additionally, if the UHMWPE piston were intentionally oversized so that its rapid entry into the UHMWPE liner constituted a conformal force fit, at times either the switch might fail to fully open due to jamming of the piston in the liner, or if it did open, either such opening could be too slow (due to high friction between the piston and the liner) to properly commutate current to the fusible element or the piston or liner could become sufficiently deformed to allow undesirable ignition product flow. One solution to this latter ignition-product-flow problem may be found in commonly assigned and filed U.S. patent application Ser. No. 524,181, filed Aug. 17, 1983 in the name of Jackson. It is another object of the present invention to eliminate the latter flow problem, the structure hereof being usable in place of or in addition to the invention of the 524,181 application.